The present invention relates to a magnetic disk drive, and more particularly to a servo control device for positioning a magnetic head on a target track of a magnetic disk.
The magnetic disk drive has a magnetic head for reading data stored on a magnetic disk or writing data onto the magnetic disk. The magnetic head is mounted in an actuator mechanism that is oscillated by a VCM (voice coil motor). When the magnetic head reads or writes data, the actuator mechanism is driven to move and position the magnetic head on a target track. Magnetic head move control is exercised in accordance with servo data, which is recorded on the magnetic disk, to position the magnetic head at a target position.
A plurality of data tracks are concentrically formed on the magnetic disk. Further, identification information and burst pattern are prerecorded in the direction of the diameter of the magnetic disk. The identification information represents the track address of each data track. In accordance with the identification information read by the magnetic head, it is possible to determine the approximate magnetic head position, that is, to determine the data track to which the magnetic head position corresponds. As regards the burst pattern, individual signal storage areas are arranged in the direction of the diameter and positioned at fixed spacing intervals, and there are a plurality of burst pattern rows, which differ in signal storage area phase. In accordance with a signal that the magnetic head generates according to a burst pattern, the precise magnetic head position can be determined. In other words, it is possible to detect the deviation (position error) of the magnetic head position from a data track to which the magnetic head corresponds.
Before data is read from or written onto the magnetic disk, the magnetic head is moved until it corresponds to a specific data track while determining the approximate magnetic head position in accordance with the identification information read by the magnetic head during magnetic disk rotation, and then the magnetic head is precisely positioned on a specific data track in accordance with a signal that the magnetic head generates according to a burst pattern. This sequence of operations is referred to as a seek operation. While data is being read or written, feedback control is exercised in accordance with a signal that the magnetic head generates according to a burst pattern so that the magnetic head is consistently positioned relative to a specific data track. This operation is referred to as a track following operation.
The magnetic disk is installed over the outer circumference of a spindle, which is rotated by a motor. In most cases, however, the center of the magnetic disk slightly deviates from the rotation center of the spindle due, for instance, to a manufacturing error. This deviation gives a particular, low-frequency disturbance to a control system that exercises feedback control to consistently position the magnetic head relative to a specific data track.
Further, the magnetic disk drive not only incurs a deviation of the rotation center of the spindle from the center of the magnetic disk, but also causes a high-frequency disturbance because, for example, the bearing of a spindle rotation motor is out of round. The high-frequency disturbance becomes a problem particularly in a case where the data track pitch on the magnetic disk is decreased (the data track width is reduced).
Patent Document 1 (Japanese Patent Laid-Open No. 328664/1996) states that the control system can be stabilized by selecting an appropriate digital filter phase when a disturbance having a known frequency is encountered. However, when a method described in Patent Document 1 is used, a disturbance having an unknown frequency cannot be handled. Patent Document 2 (Japanese Patent Laid-Open No. 109335/2003) states that when the phase of a position error signal (PES) is compared against the phase of a digital peak filter output signal to determine the phase relationship between the two signals, it is possible to ensure that the preset frequency for the digital peak filter follows the PES.